1. Field of the Invention
This invention relates to an optical resin composition which can provide a resin with a high refractive index that is well-balanced in transparency, optical distortion, thermal resistance, tintability, impact resistance, and the like; an optical resin formed by polymerization and curing thereof; and an optical lens.
2. Description of the Related Art
Now, thermosetting optical resins and their monomers which are put to practical use for a lens for eyeglasses and the like can be substantially classified into two types; a polycondensation type represented by a thiourethane resin and a radical polymerization type represented by an acrylic or vinyl compound.
A thiourethane resin has advantages such as a high refractive index and an excellent impact resistance, which has come into wide use as a resin for an optical use, mainly for an eyeglass lens. Since the resin is prepared by forming an urethane bond by means of a condensation reaction of a thiol with an isocyanate, it, however, takes a long time, e.g., more than 24 hours, to conduct polymerization while maintaining optical homogeneity. Therefore, a sulfur-containing urethane resin has excellent characteristics as a resin, but seems to remain to be improved in terms of productivity.
On the other hand, a (meth)acrylate resin is excellent in productivity, because monomers can be rapidly polymerized via a radical reaction. In terms of its physical properties as a resin, it, however, has a deadly disadvantage, i.e., low impact resistance, and also its refractive index cannot be significantly improved, except some of those prepared from a thioacrylate.
Likewise, a resin based on a polyene-polythiol reaction can be formed with a high productivity because monomers can be rapidly polymerized via a radical reaction, while it has disadvantages such as a high volume shrinkage percentage making a precise casting polymerization difficult. In addition, as for physical properties as a resin, it is generally brittle, which, of course, limits its applications. Furthermore, the refractive index of the resin can be improved by increasing a content of a thiol component in the monomer composition. However, as the content of the thiol component increases, a resin prepared by polymerization increasingly becomes rubbery and thus cannot be used for an optical product.
In brief, it can be said that there remain problems in productivity for a thiourethane resin having excellent physical properties and in physical properties for a radical polymerization type of resin with a high productivity. In order to reconcile the productivity (rapid polymerization) with physical properties, several procedures have been already reported for combining an urethane bond and a radical polymerization of a polyene, (meth)acrylate or thiol.
For example, Japanese Patent Publication No. 29692/1988 suggests a polymerizable composition comprising a prepolymer prepared from a polythiol compound and a polyisocyanate and a polyene compound, to resolve the problems of brittleness and the volume shrinkage percentage during polymerization for a resin made from only a polyene compound. It, however, discloses the composition for preparing a casting material mainly for electronic applications, but not for any optical use. Therefore, the composition cannot be used for an optical resin with a high refractive index required to be optically homogeneous. For example, among the isocyanates disclosed in the publication, tolylene diisocyanate gives a resin insufficient in light resistance; the other aromatic isocyanates give ones insufficient in Abbe number; hexamethylene diisocyanate gives one with a low refractive index; and when a mercapto carboxylate recommended as an optimal alternative in the publication is used as a thiol compound, it gives a resin with a low refractive index, which is inappropriate to achieve an optical resin with a high refractive index.
Furthermore, the publication does not pay special attention to an equivalent ratio of SH to NCO groups, just indicating a range of 1.5 to 50. However, it is the equivalent ratio that is an important parameter determining properties of a prepolymer, and should be strictly controlled. Specifically, when the ratio is too low, a prepolymer obtained may be highly viscous and thus cannot be mixed with another polyene compound, whereas when it is too high, a resulting product may not be effective as a prepolymer. In addition, it describes that a ratio between excessive SH group and an unsaturated group is generally 1:1, whereas the ratio should be a value in which the unsaturated group is sufficiently excessive to avoid a resin that tends to be rubbery. Thus, when a prepolymerized thiol is used for preparing an optical resin, the ratio between excessive SH group and an unsaturated group should be fully appropriate. In summary, monomers and their ratio should be appropriately selected; otherwise it is not feasible to provide an optical resin with a high refractive index and a high Abbe number and having excellent hardness and impact resistance.
Japanese Patent Laid-Open No. 199210/1988 and Japanese Patent Laid-Open No. 207805/1988 disclose preparation of an optical resin by means of a radical polymerization of a polyene compound having an urethane bond with a polythiol compound. However, in a reaction of a polyene with a polythiol, increase of a proportion of the polythiol may generally make a resin obtained rubbery. Thus, in order to provide a resin having adequate strength by means of reacting a polyene compound containing urethane bond with a polythiol, it is inevitable that the proportion of the polythiol in the entire resin should be considerably low. Consequently, a resin obtained may have a low sulfur content, and therefore, a resin with a high refractive index, about 1.6, cannot be prepared.
Japanese Patent Laid-Open No. 25240/1993 discloses an optical resin composition with a high refractive index comprising a mixture of a polyisocyanate and a polythiol as well as a radical-polymerizable unsaturated compound. It, however, pays no attention to an importance of prepolymerization. It is evident from the fact that the resin obtained has a low impact resistance, 21 g to 31 g, according to a dropping-ball test. To achieve an adequate effect of prepolymerization, simple blending of a polyisocyanate and a polythiol is not sufficient, but definite conditions for reacting SH with NCO groups should be selected.
The application specifies the ratio of SH to NCO groups to be 0.5 to 2. However, when a prepolymerization is fully conducted in such a ratio, a prepolymer obtained will be extremely viscous, and will not be subsequently mixed with a compound having an unsaturated polymerizable group. Therefore, the disclosed composition may be inevitably regarded as a simple mixture of an isocyanate, a thiol and a monomer having an unsaturated group. In such a case, the composition should be cured by simultaneously performing two different types of reactions, i.e., a radical reaction and an urethane condensation. Therefore, it is essential to strictly control the polymerization reaction to maintain a constant proportion of an urethane bond in a resin obtained by the polymerization. In other words, depending on polymerization conditions, SH group may be completely consumed by a radical addition before completion of a slow reaction forming an urethane bond. It is thus anticipated that the unreacted isocyanate consequently may remain in the resin obtained. Residual isocyanate group may cause significant problems such as health problems for a resin-edging worker and adverse effects on a post-processing of a lens, e.g., inconsistencies in coating or dyeing.
Furthermore, a little variation of polymerization conditions may cause varying a proportion among chemical bonds in the resin. In other words, if the polymerization conditions are not adequately controlled, it is anticipated that physical properties of the resin product may vary. In addition, since different reactions simultaneously proceed during the polymerization, attention should be adequately paid to optical strain and optical inhomogenity for the resin obtained by the polymerization. Moreover, a mold releasing agent may be needed because of an urethane condensation reaction during a casting polymerization.
Japanese Patent Laid-Open No. 228659/1995 discloses a polymerizable composition comprising a mixture of a polythiol and a polyisocyanate as well as a compound having both hydroxyl or mercapto group and an unsaturated group such as (meth)acrylic group. A polythiol having a high sulfur content as illustrated in the reference can be used to prepare a resin having a high refractive index. However, the reference does not unambiguously suggest a prepolymerization, which indicates that still the polymerization reaction should be strictly controlled. Furthermore, since the monomer having an unsaturated group has also thiol or hydroxyl groups, a resin obtained may have a high thermal resistance, but a reduced tintability.